thread

ReaderWriterMutex 

A ReaderWriterMutex is used to achieve mutual exclusion between threads, similar to a Mutex.
Unlike a Mutex, a ReaderWriterMutex can be used to gain exclusive (writer) or shared (reader)
access to what it guards. A flaw in relation to a Mutex is that it cannot be used with a condition variable.

A ReaderWriterMutex can be in one of three states:
// - Free - not owned by any thread,
// - Exclusive - owned by a single thread,
// - Shared(n) - shared amongst n threads.

 

DEBUG

trace(),assert()都只是在DEBUG的模式下才起作用的,如果定义了NDEBUG,编译器会认为是非DEBUG的模式(虽然编译出来的程序还是很大,而且还可以进行调试),此时trace(),assert()就没有用了.就如同你编译成release版的时候这些没有用一样.

 

Java中的多线程是一种抢占机制而不是分时机制。抢占机制指的是有多个线程处于可运行状态，但是只允许一个线程在运行，他们通过竞争的方式抢占CPU。

新生状态（New）： 当一个线程的实例被创建即使用new关键字和Thread类或其子类创建一个线程对象后，此时该线程处于新生(new)状态，处于新生状态的线程有自己的内存空间，但该线程并没有运行，此时线程还不是活着的（not alive）；
　　就绪状态（Runnable）： 通过调用线程实例的start()方法来启动线程使线程进入就绪状态(runnable)；处于就绪状态的线程已经具备了运行条件，但还没有被分配到CPU即不一定会被立即执行，此时处于线程就绪队列，等待系统为其分配CPCU，等待状态并不是执行状态；此时线程是活着的（alive）；
　　运行状态（Running）： 一旦获取CPU(被JVM选中)，线程就进入运行(running)状态，线程的run()方法才开始被执行；在运行状态的线程执行自己的run()方法中的操作，直到调用其他的方法而终止、或者等待某种资源而阻塞、或者完成任务而死亡；如果在给定的时间片内没有执行结束，就会被系统给换下来回到线程的等待状态；此时线程是活着的（alive）；
　　阻塞状态（Blocked）：通过调用join()、sleep()、wait()或者资源被暂用使线程处于阻塞(blocked)状态；处于Blocking状态的线程仍然是活着的（alive）
　　死亡状态（Dead）：当一个线程的run()方法运行完毕或被中断或被异常退出，该线程到达死亡(dead)状态。此时可能仍然存在一个该Thread的实例对象，当该Thready已经不可能在被作为一个可被独立执行的线程对待了，线程的独立的call stack已经被dissolved。一旦某一线程进入Dead状态，他就再也不能进入一个独立线程的生命周期了。对于一个处于Dead状态的线程调用start()方法，会出现一个运行期(runtime exception)的异常；处于Dead状态的线程不是活着的（not alive）。

1）优先级（priority）
每个类都有自己的优先级，一般property用1-10的整数表示，默认优先级是5，优先级最高是10；优先级高的线程并不一定比优先级低的线程执行的机会高，只是执行的机率高；默认一个线程的优先级和创建他的线程优先级相同；
2）Thread.sleep()/sleep(long millis)
当前线程睡眠/millis的时间（millis指定睡眠时间是其最小的不执行时间，因为sleep(millis)休眠到达后，无法保证会被JVM立即调度）；sleep()是一个静态方法(static method) ，所以他不会停止其他的线程也处于休眠状态；线程sleep()时不会失去拥有的对象锁。作用：保持对象锁，让出CPU，调用目的是不让当前线程独自霸占该进程所获取的CPU资源，以留一定的时间给其他线程执行的机会；
3）Thread.yield()
让出CPU的使用权，给其他线程执行机会、让同等优先权的线程运行（但并不保证当前线程会被JVM再次调度、使该线程重新进入Running状态），如果没有同等优先权的线程，那么yield()方法将不会起作用。
4）thread.join()
使用该方法的线程会在此之间执行完毕后再往下继续执行。
5）object.wait()
当一个线程执行到wait()方法时，他就进入到一个和该对象相关的等待池(Waiting Pool)中，同时失去了对象的机锁—暂时的，wait后还要返还对象锁。当前线程必须拥有当前对象的锁，如果当前线程不是此锁的拥有者，会抛出IllegalMonitorStateException异常,所以wait()必须在synchronized block中调用。
6）object.notify()/notifyAll()
唤醒在当前对象等待池中等待的第一个线程/所有线程。notify()/notifyAll()也必须拥有相同对象锁，否则也会抛出IllegalMonitorStateException异常。

 

//libcore/libart/src/main/java/java/lang/Thread.java

public class Thread implements Runnable {    
     /**
     * A representation of a thread's state. A given thread may only be in one
     * state at a time.
     */
    public enum State {
        /**
         * The thread has been created, but has never been started.
         */
        NEW,
        /**
         * The thread may be run.
         */
        RUNNABLE,
        /**
         * The thread is blocked and waiting for a lock.
         */
        BLOCKED,
        /**
         * The thread is waiting.
         */
        WAITING,
        /**
         * The thread is waiting for a specified amount of time.
         */
        TIMED_WAITING,
        /**
         * The thread has been terminated.
         */
        TERMINATED
    }
    
    /**
     * The maximum priority value allowed for a thread.
     * This corresponds to (but does not have the same value as)
     * {@code android.os.Process.THREAD_PRIORITY_URGENT_DISPLAY}.
     */
    public static final int MAX_PRIORITY = 10;

    /**
     * The minimum priority value allowed for a thread.
     * This corresponds to (but does not have the same value as)
     * {@code android.os.Process.THREAD_PRIORITY_LOWEST}.
     */
    public static final int MIN_PRIORITY = 1;

    /**
     * The normal (default) priority value assigned to the main thread.
     * This corresponds to (but does not have the same value as)
     * {@code android.os.Process.THREAD_PRIORITY_DEFAULT}.
     */
    public static final int NORM_PRIORITY = 5;

    /* Some of these are accessed directly by the VM; do not rename them. */
    private volatile long nativePeer; 
    volatile ThreadGroup group;
    volatile boolean daemon;
    volatile String name;    //初始化时线程的名字  default: "Thread-" + id;
    volatile int priority;
    volatile long stackSize;
    Runnable target;   //传进来的runnable
    private static int count = 0;   //计数线程组里面有几个线程

    /**
     * Holds the thread's ID. We simply count upwards, so
     * each Thread has a unique ID.
     */
    private long id;

    /**
     * Normal thread local values.
     */
    ThreadLocal.Values localValues;
    
    //group：每一个线程都属于一个group，当线程被创建时就会加入一个特定的group，当线程运行结束，会从这个 group 中移除；
    //daemon：当前线程是不是守护线程，守护线程只会在没有非守护线程运行的情况下才会运行；
    //priority：线程优先级，Java Thread 类的线程优先级取值范围为 [1, 10]，默认优先级为 5；
    //stackSize：线程栈大小，默认为 0，即使用默认的线程栈大小（由 dalvik 中的全局变量 gDvm.stackSize 决定）；
    //target：一个 Runnable 对象，Thread 的 run() 方法中会转掉该 target 的 run() 方法，这是线程真正处理事务的地方；
    //id：Android 线程 id，通过递增 count 得到该 id，如果没有显示给线程设置名字，那么就会使用 Thread+id 当作线程的名字。注意这不是真正意义上的线程 id，即在 logcat 中打印的 tid 并不是这个 id，那 tid 是指 dalvik 线程的 id；
    //localValues：线程本地存储（TLS）数据；
    
    /**
     * Reflects whether this Thread has already been started. A Thread
     * can only be started once (no recycling). Also, we need it to deduce
     * the proper Thread status.
     */
    boolean hasBeenStarted = false;

    public Thread(String threadName) {
        if (threadName == null) {
            throw new NullPointerException("threadName == null");
        }

        create(null, null, threadName, 0);
    }

    private void create(ThreadGroup group, Runnable runnable, String threadName, long stackSize) {
        Thread currentThread = Thread.currentThread();
        if (group == null) {
            group = currentThread.getThreadGroup();
        }

        if (group.isDestroyed()) {
            throw new IllegalThreadStateException("Group already destroyed");
        }

        this.group = group;

        synchronized (Thread.class) {
            id = ++Thread.count;
        }

        if (threadName == null) {
            this.name = "Thread-" + id;
        } else {
            this.name = threadName;
        }

        this.target = runnable;
        this.stackSize = stackSize;

        this.priority = currentThread.getPriority();

        this.contextClassLoader = currentThread.contextClassLoader;

        // Transfer over InheritableThreadLocals.
        if (currentThread.inheritableValues != null) {
            inheritableValues = new ThreadLocal.Values(currentThread.inheritableValues);
        }

        // add ourselves to our ThreadGroup of choice
        this.group.addThread(this);
    } 
     
    public synchronized void start() {
        checkNotStarted();
        hasBeenStarted = true;
        nativeCreate(this, stackSize, daemon);
    }
    
    public static native Thread currentThread();
    private native static void nativeCreate(Thread t, long stackSize, boolean daemon);
}

 

/art/runtime/native/java_lang_Thread.cc
static void Thread_nativeCreate(JNIEnv* env, jclass, jobject java_thread, jlong stack_size,
                                jboolean daemon) {
  Thread::CreateNativeThread(env, java_thread, stack_size, daemon == JNI_TRUE);
}

 

 

// art/runtime/native/java_lang_Thread.cc
void Thread::CreateNativeThread(JNIEnv* env, jobject java_peer, size_t stack_size, bool is_daemon) {
  CHECK(java_peer != nullptr);//即为java层的thread实例，包裹着run方法的具体实现
  Thread* self = static_cast<JNIEnvExt*>(env)->self;
  Runtime* runtime = Runtime::Current();

  // Atomically start the birth of the thread ensuring the runtime isn't shutting down.
  bool thread_start_during_shutdown = false;//这段代码用来检测thread是否在runtime宕机时start的
  {
    MutexLock mu(self, *Locks::runtime_shutdown_lock_);
    if (runtime->IsShuttingDownLocked()) {
      thread_start_during_shutdown = true;
    } else {
      runtime->StartThreadBirth();
    }
  }
  if (thread_start_during_shutdown) {//若runtime宕机了就抛出异常
    ScopedLocalRef<jclass> error_class(env, env->FindClass("java/lang/InternalError"));
    env->ThrowNew(error_class.get(), "Thread starting during runtime shutdown");
    return;
  }

  Thread* child_thread = new Thread(is_daemon);//新建子线程
  // Use global JNI ref to hold peer live while child thread starts.
  child_thread->tlsPtr_.jpeer = env->NewGlobalRef(java_peer);//把java层的run方法实体传递给子线程
  stack_size = FixStackSize(stack_size);

  // Thread.start is synchronized, so we know that nativePeer is 0, and know that we're not racing to
  // assign it.
  env->SetLongField(java_peer, WellKnownClasses::java_lang_Thread_nativePeer,
                    reinterpret_cast<jlong>(child_thread));

  pthread_t new_pthread;
  pthread_attr_t attr;
  CHECK_PTHREAD_CALL(pthread_attr_init, (&attr), "new thread");
  CHECK_PTHREAD_CALL(pthread_attr_setdetachstate, (&attr, PTHREAD_CREATE_DETACHED), "PTHREAD_CREATE_DETACHED");
  CHECK_PTHREAD_CALL(pthread_attr_setstacksize, (&attr, stack_size), stack_size);
  //创建新线程的方法，返回一个标志
  int pthread_create_result = pthread_create(&new_pthread, &attr, Thread::CreateCallback, child_thread);
  CHECK_PTHREAD_CALL(pthread_attr_destroy, (&attr), "new thread");

  //线程创建如果失败则清除子线程信息，释放空间
  if (pthread_create_result != 0) {
    // pthread_create(3) failed, so clean up.
    {
      MutexLock mu(self, *Locks::runtime_shutdown_lock_);
      runtime->EndThreadBirth();
    }
    // Manually delete the global reference since Thread::Init will not have been run.
    env->DeleteGlobalRef(child_thread->tlsPtr_.jpeer);
    child_thread->tlsPtr_.jpeer = nullptr;
    delete child_thread;
    child_thread = nullptr;
    // TODO: remove from thread group?
    env->SetLongField(java_peer, WellKnownClasses::java_lang_Thread_nativePeer, 0);
    {
      std::string msg(StringPrintf("pthread_create (%s stack) failed: %s",
                                   PrettySize(stack_size).c_str(), strerror(pthread_create_result)));
      ScopedObjectAccess soa(env);
      soa.Self()->ThrowOutOfMemoryError(msg.c_str());
    }
  }
}

 

//art/runtime/thread.cc
void* Thread::CreateCallback(void* arg) {
  Thread* self = reinterpret_cast<Thread*>(arg);
  Runtime* runtime = Runtime::Current();
  if (runtime == nullptr) {
    LOG(ERROR) << "Thread attaching to non-existent runtime: " << *self;
    return nullptr;
  }
  {
    // TODO: pass self to MutexLock - requires self to equal Thread::Current(), which is only true
    //       after self->Init().
    MutexLock mu(nullptr, *Locks::runtime_shutdown_lock_);
    // Check that if we got here we cannot be shutting down (as shutdown should never have started
    // while threads are being born).
    CHECK(!runtime->IsShuttingDownLocked());
    self->Init(runtime->GetThreadList(), runtime->GetJavaVM());
    Runtime::Current()->EndThreadBirth();
  }
  {
    ScopedObjectAccess soa(self);

    // Copy peer into self, deleting global reference when done.
    CHECK(self->tlsPtr_.jpeer != nullptr);
    self->tlsPtr_.opeer = soa.Decode<mirror::Object*>(self->tlsPtr_.jpeer);
    self->GetJniEnv()->DeleteGlobalRef(self->tlsPtr_.jpeer);
    self->tlsPtr_.jpeer = nullptr;
    self->SetThreadName(self->GetThreadName(soa)->ToModifiedUtf8().c_str());

    mirror::ArtField* priorityField = soa.DecodeField(WellKnownClasses::java_lang_Thread_priority);
    self->SetNativePriority(priorityField->GetInt(self->tlsPtr_.opeer));
    Dbg::PostThreadStart(self);

    // Invoke the 'run' method of our java.lang.Thread.
    mirror::Object* receiver = self->tlsPtr_.opeer;
    jmethodID mid = WellKnownClasses::java_lang_Thread_run;
    InvokeVirtualOrInterfaceWithJValues(soa, receiver, mid, nullptr);
  }
  // Detach and delete self.
  Runtime::Current()->GetThreadList()->Unregister(self);

  return nullptr;
}

 

//art/runtime/reflection.cc
JValue InvokeVirtualOrInterfaceWithJValues(const ScopedObjectAccessAlreadyRunnable& soa,
                                           mirror::Object* receiver, jmethodID mid, jvalue* args) {
  // We want to make sure that the stack is not within a small distance from the
  // protected region in case we are calling into a leaf function whose stack
  // check has been elided.
  if (UNLIKELY(__builtin_frame_address(0) < soa.Self()->GetStackEnd())) {
    ThrowStackOverflowError(soa.Self());
    return JValue();
  }

  mirror::ArtMethod* method = FindVirtualMethod(receiver, soa.DecodeMethod(mid));
  uint32_t shorty_len = 0;
  const char* shorty = method->GetShorty(&shorty_len);
  JValue result;
  ArgArray arg_array(shorty, shorty_len);
  arg_array.BuildArgArrayFromJValues(soa, receiver, args);
  InvokeWithArgArray(soa, method, &arg_array, &result, shorty);
  return result;
}

static void InvokeWithArgArray(const ScopedObjectAccessAlreadyRunnable& soa,
                               mirror::ArtMethod* method, ArgArray* arg_array, JValue* result,
                               const char* shorty)
    SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
  uint32_t* args = arg_array->GetArray();
  if (UNLIKELY(soa.Env()->check_jni)) {
    CheckMethodArguments(method, args);
  }

  method->Invoke(soa.Self(), args, arg_array->GetNumBytes(), result, shorty);
}

 

 inline void Thread::CheckSuspend() {
+  LOG(ERROR) << "HTC+++++ Thread::CheckSuspend()";
+  android::CallStack stack(LOG_TAG);
   DCHECK_EQ(Thread::Current(), this);

 

01-01 00:17:09.430  5961  5961 E Chameleon: HTC+++ native current thread id: 5961
01-01 00:17:09.430  5961  5961 E art     : ThreadList::SuspendAll() ...
01-01 00:17:09.430  5961  5961 E art     :  MTK_ART_OPT_ENABLE is true.
01-01 00:17:09.430  5961  6070 E art     : HTC+++++ Thread::CheckSuspend()
01-01 00:17:09.445  5961  6070 D         : #00 pc 0000000000539750  /system/lib64/libart.so (artTestSuspendFromCode+92)
01-01 00:17:09.445  5961  6070 D         : #01 pc 000000000012617c  /system/lib64/libart.so (art_quick_test_suspend+76)
01-01 00:17:09.445  5961  6070 D         : #02 pc 00000000000caab4  /data/app/com.example.chameleon-1/oat/arm64/base.odex (offset 0x103000) (void com.example.chameleon.MainActivity$2.run()+408)
01-01 00:17:09.445  5961  6070 D         : #03 pc 000000000011c924  /system/lib64/libart.so (art_quick_invoke_stub+580)
01-01 00:17:09.445  5961  6070 D         : #04 pc 000000000012c7fc  /system/lib64/libart.so (art::ArtMethod::Invoke(art::Thread*, unsigned int*, unsigned int, art::JValue*, char const*)+176)
01-01 00:17:09.445  5961  6070 D         : #05 pc 000000000042d48c  /system/lib64/libart.so (art::InvokeVirtualOrInterfaceWithJValues(art::ScopedObjectAccessAlreadyRunnable const&, _jobject*, _jmethodID*, jvalue*)+440)
01-01 00:17:09.445  5961  6070 D         : #06 pc 000000000045c6ac  /system/lib64/libart.so (art::Thread::CreateCallback(void*)+756)
01-01 00:17:09.445  5961  6070 D         : #07 pc 000000000006a044  /system/lib64/libc.so (__pthread_start(void*)+52)
01-01 00:17:09.445  5961  6070 D         : #08 pc 000000000001c844  /system/lib64/libc.so (__start_thread+16)